The invention relates to a process and a circuit arrangement for the compensation of echo signals in signal transmission equipment for the duplex transmission of digital signals. Such transmission equipment is connected in a duplex channel system with a two-wire line and each unit includes a sending device and a receiving device, with, on the one hand, compensation signals being generated in each case by an echo compensator for the compensation of the echo signals that occur in the associated receiving device when digital signals are fed by the sending device of a signal transmission unit to the two-wire line. These compensation signals are subtracted in the receiving device of the signal transmission unit in question from the digital data signals that are received with the echo signals at scanning times that occur in uniform time intervals, and, on the other hand, with adjustment data being fed by the receiving device to a timing generator in order to regulate the phase position of the clock pulses of a clock pulse sequence which determine the scanning times by their occurence.
It is already a known procedure to arrange for the compensation of echo signals in signal transmission equipment for the duplex transmission of digital signals, which is connected in a duplex channel system with a two-wire line, by providing an echo compensator in each case, which generates compensation signals that are patterned on the time sequence of the echo signals and are subtracted from the digital signals received with the echo signals. For this purpose it is possible, for example, when the digital signals are fed from one of the signal transmission units to the two-wire line, to form in the associated echo compensator an echo compensator value for each signal bit that is fed in, which is subtracted from a signal bit of the digital signals received with the echo signals that is scanned in the receiving device at a certain moment. This presupposes that the scanning times determined for scanning the individual signal bits have an extremely high constancy. Even small deviations from the scanning times can produce large, uncompensated echo components, since the echo signals that occur in a signal transmission unit when digital signals are transmitted over substantial distances can be significantly greater than the actual digital signals that are fed to the signal transmission unit in question as receiver signals. The high constancy of the scanning times that is required for the interference-free operation of signal transmission equipment of this kind can then be achieved by providing in each unit of the signal transmission equipment a timing generator which digitally corrects even the slightest fluctuations of the phase position of the clock pulses that determine the scanning times by their occurence. For this purpose, it is necessary to include a shift pulse in order to shift the phase position, whose frequency is significantly higher than the frequency of the digital signals to be transmitted. If the signal transmission speed is high, this may mean that the shift pulse can no longer be processed with conventional integrated circuits in digital technology.